Unconventional natural gas source includes coal-bed gas, shale gas, tight sandstone gas and water-soluble gas. Currently, the unconventional oil and gas resources have been played an important role in the global energy structure. In this country, the unconventional oil and gas resources have rich reserves, wide distribution and broad development prospects. The technical exploitable resource of the unconventional natural gas is up to 3.4×105 billion cubic meters, which is 1.8 times of the technical exploitable resource of the conventional natural gas.
However, the unconventional oil and gas resources are difficult to exploit and basically have characteristics of low porosity, low permeability, low pressure, and compact rock formation. Historical breakthrough of the shale exploitation technology is realized by combining the horizontal drilling technology and horizontal fracturing technology with the Barnett shale in Fort Worth Basin in Texas, US, and the single well exploitation cost is greatly reduced. Therefore, for the unconventional oil and gas reservoir, fracturing manner is mainly used for transform the oil and gas reservoir to improve the oil (gas) productivity index, thereby improving single well yield and stable yield validity. The conventional water base fracturing causes large damage to the shale oil and gas reservoir and causes severe secondary pollution in the solution, and severely affects the capacity increasing revamping effect of the solution, specifically in the following aspects: (1) damage to the stratum caused by the fracturing fluid polymer; (2) damage caused to the stratum due to unreasonable drainage speed; and (3) damage easily caused to the water sensitive stratum.
The supercritical carbon dioxide injection fracturing includes producing a supercritical carbon dioxide jet flow by the injection fracturing apparatus, injecting through the casing and the reservoir rock to form a perforation hole and then performing fracturing. The supercritical carbon dioxide injection fracturing has the following characteristics: no free water, conveying proppant agents to form propped fractures, no free water and solid residues in the propped fractures, being completely suitable for water sensitive reservoir and removing the damage to the reservoir. Therefore, the supercritical carbon dioxide injection fracturing can improve the flow conductivity of the unconventional oil and gas reservoir and is one of the effective means for realizing effective commercial exploitation. The supercritical carbon dioxide fracturing fluid is a mixed substance formed by dissolving the thickening agent into the supercritical carbon dioxide. The fracturing fluid flows by the injection fracturing apparatus to create throttling effect, causing the temperature to reduce and affecting the physical property and injection fracturing effect of the fracturing fluid. Currently, with the influence of the temperature, pressure and the viscosity of the fracturing fluid, the rules of the throttling effect of the supercritical carbon dioxide fracturing fluid are relatively complex with few test data. The viscosity of the supercritical carbon dioxide fracturing fluid changes with the temperature and pressure. There is no systematic explanation for the principle of the throttling process, so that it is a difficulty in the supercritical carbon dioxide injection fracturing exploitation of the unconventional oil and gas reservoir.